The mucosa associated lymphoid tissue (MALT) appears as intra epithelial lymphoid region. Majority of cells in this MALT are T cells.
The lamina propria that lies beneath the epithelial layer contains large number of B cells, activated TH cells, and macrophages in the form of loose clusters.
The sub mucosa layer beneath the lamina propria contains payer’s patches, (nodules of 30-40 lymphoid follicles). A specialized cell group known as M cells (microfold cells) are present in mucous membrane to carry the antigenes from surface to the under lying mucosa associated lymphoid tissue.
Analogous to the function, structure of M cells is also different; unlike epithelial cells of mucosal layer M cells are flat without any microvillie.
The baso lateral membrane of the cells form pockets and accommodate clusters of B cells, T cells and macrophages in it. The phagocytes formed from the surface with antigen fuse with the membrane of pockets and release the phagocytized antigen in to it, for immunological reactions.
The activated B cells differentiate in to plasma cells and secrete Ig A antibodies. These antibodies then transported out to the lumen through epithelial cells as secretory Ig A, to interact with the antigens in the surrounding areas of lumen.
Like mucosal layer the skin also is provided with lymphocytes for primary defense. The Keratinocytes (epithelial cells specialized for immunity) are capable of inducing inflammation with their secretions in the presence of a pathogen.
The dendritic langerhan cells of epidermis can engulf pathogen and move to regional lymph nodes to present the antigen to TH cells. The intra epidermal lymphocytes also check the pathogen entering through the skin. The vermiform appendix is also very rich in lymphoid tissue and is considered as “abdominal tonsil”.